Various kinds of golf balls are commercially selling, because various kinds of characteristics, such as long flight distance, excellent controllability with approach shots, and good shot feel at the time of hitting, are required by golfers. Amateur golfers generally desire and require long flight distance and good shot feel in the characteristics of golf balls and therefore tend to employ golf balls which are soft and have long flight distance. In order to extend the flight distance, it is required to restrain the spin amount to be as low as possible. It is also required to increase the deformation amount of the golf ball, in order to obtain a soft and good shot feel.
Golf balls are typically classified into thread wound golf balls and solid golf balls (e.g. two-piece golf ball, three-piece golf ball and the like). The thread wound golf balls have soft shot feel in comparison with the solid golf balls. In order to impart shot feel equal to the thread wound golf balls to the solid golf balls, many improvements have recently been made to the solid golf balls. However, the thread wound golf balls inherently have good shot feel because of the presence of a soft thread wound layer and the improvement of shot feel on the solid golf balls has not been sufficiently accomplished.
The thread wound golf ball is generally composed of a center, a thread wound layer formed on the center and a balata cover formed on the thread wound layer. The balata-covered thread wound golf ball has very good shot feel, but has poor durability, so that the cover is easily cut by an iron shot. Instead of the balata cover, an ionomer cover has also been proposed to improve durability. The use of the ionomer cover properly improves the durability of the thread wound golf ball and also imparts longer flight distance than the balata-covered thread wound golf ball. However, the ionomer-covered thread wound golf ball adversely affects the shot feel inherent to the thread wound golf ball.
In order to impart good durability to the thread wound golf ball while maintaining soft and good shot feel, it has been proposed in Japanese Patent Kokai Publication Nos. 98902/1996, 224323/1996, 332248/1996, 173505/1997 and the like that the cover of the thread wound golf ball is formed into two-layer structure, i.e., inner cover layer and outer cover layer. In the proposed golf balls, the inner cover layer is made harder and the outer cover layer is made softer, to achieve a balance of spin amount and durability. However, when the outer cover layer is softer than the inner cover layer, the resultant golf ball has high durability, but has high spin amount, which results in a reduction of flight distance.